A wireless cellular communication system typically comprises one or several radio carriers, on which information is transmitted. The bandwidth of a radio carrier would have to be chosen such that it can be accommodated within the spectrum resources the system operator has at its disposal. The spectrum allocated to a certain system operator is often a consequence of national regulatory decisions and may be technology neutral.
Unfortunately, the carrier bandwidths that radio communications systems can provide have always limited flexibility. For example, the 3GPP Long Term Evolution (LTE) standard (E-UTRA) uses Orthogonal Frequency Division Multiplex (OFDM) transmission which in principle allows for very flexible transmission bandwidth by configuring a suitable number of OFDM subcarriers. However, for each bandwidth configuration it is needed to specify performance requirements for various channels, spectral masks for out-of-band emissions, test cases for transmitters and receivers, etc, which increases the cost and complexity of the equipment. Thus making a large number of bandwidth configurations specified by a standard is not practical. This is the reason why only six channel bandwidths are currently supported in LTE: 20, 15, 10, 5, 3 and 1.4 MHz.
In a number of real-life LTE deployment cases it has been already noticed that these bandwidths do not perfectly match the spectrum allocations available to the operators. For example, if 19 MHz is available, the largest single-carrier LTE bandwidth that can be deployed is 15 MHz. The remaining 4 MHz can either not be used for LTE or can be partially used by deploying an additional LTE carrier with 3 MHz carrier bandwidth. Multiple carriers can be aggregated for transmission and reception to one user. This co-called carrier aggregation solution is specified in Rel. 10 of the LTE standard, which however demands special categories of mobile terminals. If a terminal is not capable of carrier aggregation, it would have to use either the 3 MHz or the 15 MHz carrier, which will limit its maximum throughput.
The additional problem for the operators is that not all carrier bandwidths are supported by the LTE standard in all frequency bands, so carrier aggregation will not always be possible. In the given example a 3 MHz LTE carrier might not be possible to deploy in the remaining spectrum because all the carrier bandwidths are not defined in all frequency bands. Even if carrier aggregation is used as in the above example, there could still be left-over spectrum that is not used, so this obviously is not the most efficient solution. Thereto, with carrier aggregation, parts of the spectrum have to be used as guard band between carriers.
Introducing new transmission bandwidth configurations could provide a way to use the available bandwidth more efficiently. However, set aside the issues with standardization and testing, it would also be a problem to determine suitable values for new bandwidth configurations that would fit most system deployments world wide. It should also be noted that new bandwidth configurations will not be accessible to terminals presently in use, but only to terminals of releases of the system, for which the new bandwidth configuration has been introduced. Hence, introducing new transmission bandwidth configurations poses problems for system operators that already have deployed a system, as different carriers will be accessible to terminals of different system releases.
Two types of solutions for improving the spectrum utilization have been proposed in the past for the prior art LTE system: carrier segments and extension carriers.
The carrier segments are contiguous bandwidth extensions to a normal LTE carrier. This solution implies that the normal LTE carrier bandwidth is smaller than the available amount of spectrum, so that the segments can be deployed in the remaining parts. The segments can be used either for user data transmission or for the transmission of some new control channels that might be defined in the future releases of the standard. The sum of the channel bandwidths of the normal LTE carrier and the segments cannot be larger than 20 MHz, because one control channel (located on the normal LTE carrier) is used for scheduling transmission on both the normal LTE carrier and the segments, while the control channel of the normal LTE carrier cannot handle resource allocation for larger channel bandwidths. Since only one control channel is used, only one Hybrid Automatic Repeat reQuest (HARQ) process is utilized, and the same transmission mode is used on the segment as on the normal LTE carrier. It has been proposed that the size of the segments is limited to be the same as the channel bandwidths supported in LTE; 20, 15, 10, 5, 3 and 1.4 MHz.
The extension carrier is defined as a supplementary component carrier to the normal LTE carrier, which serves only for user data traffic transmission. The corresponding control information is transmitted over the control channels allocated on a normal LTE carrier. It was also suggested that an extension carrier or a carrier segment does not include broadcast channels, synchronization signals and the common reference signals (CRS). It means that extension carrier cannot be operated stand-alone and must be part of carrier aggregation. As opposed to carrier segments, there is no restriction on the sum of normal LTE carrier bandwidth and extension carrier bandwidth, except that each of them can be at most 20 MHz. Furthermore, an extension carrier does not need to be located contiguously to the normal LTE carrier. The extension carrier is scheduled from the normal LTE carrier but using a separate control channel, i.e., there is a separate control channel for scheduling transmissions on an extension carrier and another one for scheduling transmissions on a normal LTE carrier. Since the extension carrier has a separate control channel, it also has a separate HARQ process, and different transmission modes can be used on the extension carrier and the normal LTE carrier. It has been proposed that the extension carrier bandwidth can be configured the same as the channel bandwidths supported in LTE; 20, 15, 10, 5, 3 and 1.4 MHz.
However, when the bandwidth of an available spectrum resource does not match a combination of supported bandwidths, there is still a waste of such spectrum resources that will be left unused. For instance, in the case of LTE, a spectrum resource of 19 MHz configured with a component carrier of 15 MHz and a carrier segment or extension carrier of 3 MHz still leaves 1 MHz unused.